Getting grains and fermenting it yourself is the cheapest and by far the best way to consume kefir. Studies of probiotic yoghurt and capsules have shown that they rarely contain anywhere near the “at time of production” figure stated on the pack. I don’t see why kefir would be any different. I’m pretty sure it’s made from some sort of starter culture rather than the original grains, and doesn’t contain the amount of strains real kefir would, which can number in the dozens. So you’re left paying more for a product with a 1/3 of the strains and a fraction of the CFUs of a homemade batch, and probably more lactose to boot. You’ll never have any surplus grains to eat or experiment with (coconut milk kefir?) either.